Technique for extracorporeal treatment of blood

Abstract

Blood from a patient is treated extracorporeally by any selected one of a plurality of predetermined treatments. Each of the treatments involves the flow of blood from the patient into a primary chamber of a filtration unit, past a semipermeable membrane located in the filtration unit which separates the primary chamber from a secondary chamber of the filtration unit, out the filtration unit and back to the patient. A replacement fluid is selectively and controllably added to the blood, as required for the selected treatment. A secondary fluid is controllably and selectively introduced into the secondary chamber of the filtration unit for controllably collecting material passing across the semipermeable membrane from the blood or for supplying material to pass across the semipermeable membrane into the blood as required for the selected treatment. Materials are removed from the secondary chamber and collected in accordance with the selected treatment. Upon selection of one of the plurality of extracorporeal blood treatments available, the rates of blood flow, fluid flow and fluid collection during the extracorporeal treatment are automatically determined, established and monitored in accordance with the selected treatment. An apparatus which automatically performs and monitors the selected treatment is also described.

We claim: 
     
       1. An apparatus for treating blood from a patient extracorporeally by a treatment which transfers matter to or from the blood through a semipermeable membrane in a filtration unit, the semipermeable membrane dividing the filtration unit into a primary chamber and a secondary chamber, the primary chamber receiving the blood and the matter diffused across the semipermeable membrane into the blood, the secondary chamber receiving the matter to be diffused into the blood and the matter transferred across the semipermeable membrane from the blood, said apparatus comprising: means connected to the primary chamber and operative for establishing an extracorporeal blood flow primary circuit from the patient through the primary chamber and back to the patient;   a primary pump connected in the extracorporeal circuit and operative for controlling the flow of blood through the primary circuit;   a replacement fluid supply connected in the primary circuit and operative for supplying a replacement fluid to the blood in the primary circuit;   a replacement fluid pump coupled to the replacement fluid supply and operative for controlling the amount of replacement fluid supplied to the blood in the extracorporeal circuit;   means connected to the secondary chamber for establishing a secondary circuit for the flow of fluid through the secondary chamber;   a secondary fluid supply connected to the secondary circuit and operative for supplying one of a secondary fluid containing matter which is to be diffused across the semipermeable membrane into the blood or a secondary fluid to remove matter from the secondary chamber which has been transferred across the semipermeable membrane from the blood;   a secondary fluid pump coupled to the secondary fluid supply and operative for controlling the amount of secondary fluid supplied to the secondary chamber;   a collection reservoir connected to the secondary flow circuit and operative for collecting fluid exiting from the secondary chamber;   a collection pump coupled to the collection reservoir and operative for controlling the amount of fluid collected from the secondary chamber;   an input device for selecting one of a plurality of different extracorporeal blood treatments to be performed by the apparatus from the group consisting of UF, HD, HF, and HDF;   a memory for storing predefined flow rate information for each of the plurality of different extracorporeal blood treatments, including information defining the flow rates in relationship to the treatment selected and for storing information describing the ones of the replacement fluid and the secondary supply fluid which are to be selectively supplied and the collection fluid which is to be selectively collected for each type of treatment, and for accumulating and storing flow rate information about an individual patient being treated with a selected treatment throughout the course of continuous and interrupted treatment; and   a controller coupled to the memory, the input device, the primary pump, the replacement fluid pump, the secondary fluid pump and the collection pump and operative in response to the selection of each type of treatment for automatically controlling the rate of flow of fluid through the primary circuit, the rate of flow of fluid from the replacement fluid supply through the replacement fluid pump into the primary circuit, the rate of flow of fluid from the secondary fluid supply through the secondary fluid pump into the secondary chamber and the rate of flow of fluid through the collection pump from the secondary chamber according to the type of treatment selected and the flow rate information stored in the memory which relates to the type of treatment selected.   
     
     
       2. The apparatus as defined in claim 1 wherein the input device is further operative for selecting a flow rate for the treatment selected which is different than one of the stored flow rates relating to the treatment selected. 
     
     
       3. The apparatus as defined in claim 1 wherein; the controller is further operative for correlating the selected flow rate with the stored flow rate and for adjusting the stored flow rate to effect treatment of the blood in accordance with the treatment and the flow rate selected.   
     
     
       4. The apparatus as defined in claim 1 further comprising: gravimetric scales connected to the replacement fluid supply and the secondary fluid supply and the collection reservoir and operative for providing redundant weight information relative to the amount of each fluid supplied and collected; and wherein:   the controller is connected to the gravimetric scales and is operative for controlling the rates of fluid flow from the pumps in response to the weight information.   
     
     
       5. The apparatus as defined in claim 4 further comprising: monitor means connected to the controller and connected to the gravimetric scales and operative for deriving from the weight information, weight information relative to the amount of each fluid supplied and collected and for producing comparisons of the amount of each fluid supplied and collected with amounts expected according to the treatment selected.   
     
     
       6. The apparatus as defined in claim 5 wherein: the monitor means provides the controller with the results of said comparisons; and   the controller operatively controls the flow rates established by the pumps in response to said comparisons.   
     
     
       7. The apparatus as defined in claim 5 further comprising: a detector connected to the monitor means and to one or both of the primary and secondary circuits and operative for providing information about conditions of the blood and the fluids in the primary and secondary circuits.   
     
     
       8. The apparatus as defined in claim 7 wherein the detector further comprises: a bubble detector connected to the primary circuit and to the monitor means and operative for detecting bubbles and microbubbles in the blood in the primary circuit.   
     
     
       9. The apparatus as defined in claim 1 wherein the controller is further operative prior to treatment in accordance with the treatment selected to automatically prime the primary and secondary circuits by selectively moving priming fluid in the primary circuit for a predetermined period of time, ultrafiltering the priming fluid in the primary circuit through the primary chamber and into the secondary chamber, fully priming portions of the primary and secondary circuits to be used during the selected treatment, and partially filling with priming fluid portions the primary and second circuits not to be used during the selected treatment. 
     
     
       10. The apparatus as defined in claim 1 wherein said memory includes a shared memory connected to said controller, said apparatus further comprising: a monitor connected in the primary and secondary circuits and associated with the pumps to receive monitor signals related to the performance of the pumps, the monitor operative for determining when an alarm condition has occurred in response to monitor signal, said monitor means further connected to said shared memory, said shared memory operative for accepting, storing and transferring information between the controller and the monitor.   
     
     
       11. The apparatus as defined in claim 10 wherein the alarm condition is one selected from a group consisting of failure of a bubble detector connected in the primary circuit, failure of a leak detector connected in the secondary circuit, operation of a pump when the operation of the pump has been signaled by the controller to halt, pumping by a pump at a rate different than that directed by the controller, failure to operate by a pump when the pump has been directed to operate by the controller, failure to obtain a pressure pulse from a patient by a pressure detector connected in the primary circuit, and failure to detect a pressure from a pressure detector connected in the secondary circuit. 
     
     
       12. The apparatus as defined in claim 10 wherein the alarm condition is one selected from a group consisting of inconsistent signals received by the controller and monitor means about the weight of the secondary fluid, inconsistent measurement signals received by the controller and monitor means about the weight of the replacement fluid, inconsistent signals received by the controller and monitor means about the weight of the collection fluid, an indication received by the monitor means that secondary fluid is not available, an indication received by the monitor means that replacement fluid is not available, an indication received by the monitor means that collection fluid has been collected in excess of a predetermined amount, and an indication received by the monitor means that the rate of collection of collection fluid is in excess of a predetermined rate. 
     
     
       13. The apparatus as defined in claim 10 wherein the alarm condition is a monitor signal received by the monitor selected from a group indicating that there is a low blood flow rate in the primary circuit, the secondary fluid supply is empty, the replacement fluid supply is empty, the collection reservoir is full, an anticoagulant supply connected in the primary circuit is empty, a scale for weighing the secondary fluid supply has been disturbed, a scale for weighing the replacement fluid supply has been disturbed, a scale for weighing the collection fluid reservoir has been disturbed, a power loss has been detected, the apparatus requires maintenance, the semipermeable membrane is beginning to clog, the filtration unit requires replacement, treatment has been halted for more than a predetermined amount of time, pressure in the primary circuit has fallen below a previously determined point, and pressure in the primary circuit has risen above a previously determined point. 
     
     
       14. The apparatus as defined in claim 1 wherein the plurality of treatments further includes SCUF, CVVHF, CVVHD and CVVHDF. 
     
     
       15. The apparatus as defined in claim 1 further comprising: monitor means coupled to the controller, the primary circuit, the secondary circuit and the pumps, and operative to receive monitor signals related to performance of the pumps and conditions of the fluids in the primary and secondary circuits and to determine when an alarm condition has occurred.   
     
     
       16. The apparatus as defined in claim 15 further comprising: a display coupled to the controller and the monitor means and operative to display information about an alarm condition which has occurred and the steps to remedy the alarm condition.   
     
     
       17. The apparatus as defined in claim 15 wherein the alarm condition which has occurred is one of a plurality of alarm conditions, each of which is a member of one of a plurality of alarm condition classes, and the apparatus is further operative to determine in which alarm condition class the alarm condition which has occurred belongs, and wherein the apparatus further comprises: a display coupled to the controller and the monitor means and operative to display an indication of which alarm condition class to which the alarm condition which has occurred belongs.   
     
     
       18. The apparatus as defined in claim 17 wherein a different audible alarm is associated with each of the alarm condition classes. 
     
     
       19. The apparatus as defined in claim 17 wherein the alarm condition classes include (a) a first class of alarm conditions in which all pumps are automatically stopped, the apparatus must be disconnected from the patient and the alarm condition remedied before the selected treatment may be resumed, (b) a second class for alarm conditions in which all pumps are automatically stopped and the treatment may be resumed without disconnecting the apparatus from the patient after the alarm condition is remedied; (c) a third class of alarm conditions in which the alarm may be overridden and treatment may proceed without remedying the alarm condition; and (d) a fourth class of alarm conditions in which the apparatus indicates an alarm condition and treatment continues uninterrupted. 
     
     
       20. The apparatus as defined in claim 19 wherein a different audible alarm is associated with each of the alarm condition classes. 
     
     
       21. The apparatus as defined in claim 19 wherein the display is further operative to display detailed information about the alarm condition and the steps to remedy the alarm condition. 
     
     
       22. The apparatus as defined in claim 21 wherein a different audible alarm is associated with each of the alarm condition classes. 
     
     
       23. A method for treating blood from a patient extracorporeally by a treatment which transfers matter to or from the blood through a semipermeable membrane, the semipermeable membrane being part of a filtration unit of an apparatus and dividing the filtration unit into a primary chamber and a secondary chamber, the primary chamber receiving the blood and the matter transferred across the semipermeable membrane into the blood, the secondary chamber receiving the matter to be transferred into the blood and the matter transferred across the semipermeable membrane from the blood, said method comprising the steps of: establishing an extracorporeal blood flow primary circuit from the patient through the primary chamber and back to the patient;   controlling the flow of blood through the primary circuit;   selectively supplying a replacement fluid to the blood in the primary circuit prior to returning the blood to the patient;   controlling the amount of the replacement fluid supplied to the blood in the primary circuit;   establishing a secondary circuit for the flow of fluid through the secondary chamber;   selectively supplying a one of a secondary fluid containing matter which is to be transferred across the semipermeable membrane into the blood or a secondary fluid to remove matter from the secondary chamber which has been transferred across the semipermeable membrane from the blood to the secondary chamber;   controlling the amount of secondary fluid supplied to the secondary chamber;   collecting fluid exiting from the secondary chamber;   controlling the amount of fluid collected from the secondary chamber;   selecting one of a plurality of different extracorporeal blood treatments to be performed by the apparatus from the group consisting of UF, HD, HF, and HDF;   storing predefined information defining which of the ones of the replacement fluid and the secondary fluid are to be supplied and defining the flow rates in the primary and secondary circuits for each of the plurality of different extracorporeal blood treatments,   accumulating and storing flow rate information about an individual patient being treated with a selected treatment throughout the course of continuous and interrupted treatment; and   automatically adjusting and controlling the rate of flow of fluid through the primary circuit, the rate of flow of replacement fluid into the primary circuit, the rate of flow of secondary fluid into the secondary chamber and the rate of flow of fluid collected from the secondary chamber according to the extracorporeal blood treatment selected and the stored predefined information relating to the type of treatment selected.   
     
     
       24. The method as defined in claim 23 wherein the apparatus further includes a display and the method further includes the step of: displaying information regarding the ones of the replacement and secondary fluids to be selectively supplied and the collection fluid to be selectively collected and the rates of fluid flow in the primary and secondary circuits.   
     
     
       25. The method as defined in claim 24 further including the step of: selecting a flow rate for the treatment selected which is different than one of the stored flow rates relating to the treatment selected.   
     
     
       26. The method as defined in claim 25 further including the steps of: correlating the selected flow rate with the stored flow rate; and   adjusting the stored flow rate to effect treatment of the blood in accordance with the treatment and the flow rate selected.   
     
     
       27. The method as defined in claim 23 wherein the apparatus further includes gravimetric scales and the method further includes the steps of: obtaining redundant weight information from the gravimetric scales about the amount of each of the ones of replacement and secondary fluid supplied and the fluid collected from the secondary chamber; and   controlling the rates of fluid flow in the primary and secondary circuits in response to the weight information.   
     
     
       28. The method as defined in claim 27 further including the steps of: automatically deriving from the weight information the amount of each fluid supplied and collected; and   automatically comparing the information relative to the amount of each fluid supplied and collected with amounts expected according to the treatment selected.   
     
     
       29. The method as defined in claim 28 further including the step of: providing information about the pressure of the blood and fluids in the primary and secondary circuits.   
     
     
       30. The method as defined in claim 28 further including the step of: detecting bubbles in the blood in the primary circuit.   
     
     
       31. The method as defined in claim 23 further including the steps of: determining when an alarm condition has occurred in response to the weight and pressure information, wherein the alarm condition is one selected from a group consisting of failure of a bubble detector means connected in the primary circuit, failure of a blood leak detector means connected in the secondary circuit, operation of a pump means when the operation of the pump has been signaled by the controller means to halt, pumping by a pump means at a rate different than that directed by the controller means, failure to operate by a pump means when the pump means has been directed to operate by the controller means, failure to obtain a pressure pulse from a patient by a pressure detector connected in the primary circuit, and failure to detect a pressure from a pressure detector connected in the secondary circuit; and   displaying information describing the alarm condition.   
     
     
       32. The method as defined in claim 23 wherein the apparatus includes a control computer and a monitor computer, the method further including the steps of: accepting information from the control computer and the monitor computer;   storing the accepted information;   transferring the stored information from the control computer to the monitor computer and from the monitor computer to the control computer;   determining when an alarm condition has occurred in response to the weight and pressure information, wherein the alarm condition is one selected from a group consisting of inconsistent information from the control and the monitor computers about the weight of the secondary fluid, inconsistent information from the control and monitor computers about the weight of the replacement fluid, inconsistent information in the control and monitor computers about the weight of the collection fluid, an indication that secondary fluid is unavailable, an indication that replacement fluid is unavailable, an indication that collection fluid has been collected in excess of a predetermined amount, and an indication that the rate of collection of collection fluid is in excess of a predetermined rate; and   displaying information describing the alarm condition.   
     
     
       33. The method as defined in claim 23 wherein the apparatus includes a control computer and a monitor computer, the method further including the steps of: obtaining redundant weight information;   storing the redundant weight information in the control computer and in the monitor computer;   determining when an alarm condition has occurred in response to the weight and pressure information, wherein the alarm condition is selected from a group indicating that there is a low blood flow rate in the primary circuit, a secondary fluid container is empty, a replacement fluid container is empty, a collection fluid container is full, an anticoagulant container is empty, a scale for weighing the secondary fluid supply means has been disturbed, a scale for weighing the replacement fluid container has been disturbed, a scale for weighing the collection fluid container has been disturbed, a power loss has been detected, the apparatus requires maintenance, the semipermeable membrane is beginning to clog, the filtration unit requires replacement, treatment has been halted for more than a predetermined amount of time, pressure in the primary circuit has fallen below a previously determined point, and pressure in the primary circuit has risen above a previously determined point; and   displaying information describing the alarm condition.   
     
     
       34. The method as defined in claim 23 wherein the plurality of treatments available to be selected includes SCUF, CVVHF, CVVHD and CVVHDF. 
     
     
       35. Method for controlling and monitoring the circulation of body fluids and medical fluids through a circuit for a plurality of different blood extracorporeal treatments, said circuit having a memory operatively associated therewith in which is stored (a) a plurality of sets of predefined flow rates with at least one of said sets associated with each said different treatment, and (b) priming information about said different treatments, said circuit comprising: a filtration unit having a primary and a secondary chamber separated by a semipermeable membrane, each chamber having an inlet and an outlet;   a primary circuit comprising a blood access line connected to the inlet of the primary chamber and a blood return line connected to the outlet of the primary chamber;   a secondary circuit comprising a secondary fluid line connected to the inlet of the secondary chamber and a collection fluid line connected to the outlet of the secondary chamber;   a replacement fluid line connected to the primary circuit; and   an anticoagulant fluid line connected to the blood access line; the method comprising the steps of: selecting a specific treatment from a group of different extracorporeal blood treatments consisting of UF, HD, HF, and HDF, said specific treatment having a set of predefined flow rates which is one of the plurality of sets stored in the memory operatively associated with the circuit;   automatically priming the primary circuit and at least one part of the secondary circuit in accordance with the selected treatment and the priming information about the selected treatment stored in the memory;   connecting the primary circuit to a vascular system of a patient;   circulating blood, anticoagulant fluid, replacement fluid, secondary fluid and collection fluid in the circuit for multiple blood extracorporeal treatments according to the corresponding flow rates previously defined;   monitoring a pressure in at least one of the blood access line, the blood return line and the collection fluid line;   monitoring a flow rate of liquid in at least one of the primary circuit, the anticoagulant fluid line, the replacement fluid line, the secondary fluid line and the collection fluid line; and   stopping the circulation of at least one circulating fluid in the circuit for multiple extracorporeal blood treatments when predetermined alarm conditions occur.     
     
     
       36. Method according to claim 35, wherein the step of defining the flow rate of the body fluids and the medical fluids comprise the step of calculating the flow rate of the collection fluid according to the equation:   W=WL+D+R+AC     where:   WL=flow rate of the body fluid to be removed from the patient;   R=flow rate of the replacement fluid;   D=flow rate of the secondary fluid; and   AC=flow rate of the anticoagulant fluid.   
     
     
       37. Method according to claim 36, wherein the step of monitoring the flow rate of the collection fluid comprises: measuring at regular intervals the actual weight of a container connected to the collection fluid line,   for each interval, calculating a desired weight for at least one of the fluid to be removed from the patient, the secondary fluid, the replacement fluid and the anticoagulant fluid, from the corresponding defined flow rate and the treatment time elapsed;   for each interval, calculating a desired weight for the collection container from the desired weight of at least one of the fluid to be removed from the patient, the secondary fluid, the replacement fluid and the anticoagulant fluid;   calculating the difference between the actual weight and the desired weight of the collection fluid container;   regulating the circulation of the collection fluid in order that the difference between the actual weight and the desired weight of the collection fluid tends towards zero.   
     
     
       38. Method according to claim 35, wherein the step of defining the flow rate of the body fluids and the medical fluids comprise the step of calculating the flow rate of the collection fluid according to the equation:   W=WL +D +R     where:   WL=flow rate of the body fluid to be removed from the patient;   R=flow rate of the replacement fluid; and   D=flow rate of the secondary fluid.   
     
     
       39. Method according to claim 38, wherein the step of monitoring the flow rate of the collection fluid comprises: measuring at regular intervals the actual weight of a container connected to the collection fluid line,   for each interval, calculating a desired weight for at least one of the fluid to be removed from the patient, the secondary fluid, the replacement fluid and the anticoagulant fluid, from the corresponding defined flow rate and the treatment time elapsed;   for each interval, calculating a desired weight for the collection container from the desired weight of at least one of the fluid to be removed from the patient, the secondary fluid, the replacement fluid and the anticoagulant fluid;   calculating the difference between the actual weight and the desired weight of the collection fluid container;   regulating the circulation of the collection fluid in order that the difference between the actual weight and the desired weight of the collection fluid tends towards zero.   
     
     
       40. Method according to claim 35, wherein the step of priming the primary circuit and at least one part of the secondary circuit comprises: circulating a priming fluid from an end of the blood return line to an end of the blood access line and through the primary chamber, for a predetermined priming time; and   when the primary circuit is filled with priming fluid, ultrafiltering priming fluid from the primary chamber into the secondary chamber.   
     
     
       41. Method according to claim 40, further comprising: circulating priming liquid from the secondary chamber to an end of the collection fluid line.   
     
     
       42. Method according to claim 40, further comprising: when the primary circuit is filled with priming fluid, pumping priming fluid from the primary circuit into the replacement fluid line in order to fill, at least partially, the replacement fluid line with priming fluid.   
     
     
       43. Method according to claim 40, further comprising: when the secondary chamber is filled with priming fluid, pumping priming fluid from the secondary chamber into the secondary fluid line in order to fill, at least partially, the secondary fluid line with priming fluid.   
     
     
       44. Method according to claim 40, further comprising: priming the replacement fluid line by circulating replacement fluid in the replacement fluid line for a predetermined fraction of the priming time.   
     
     
       45. Method according to claim 40, further comprising: priming the secondary fluid line by circulating secondary fluid in the secondary fluid line for a predetermined fraction of the priming time.   
     
     
       46. Method according to claim 40, further comprising, when the priming time has elapsed, the steps of: closing the blood return line;   pumping priming fluid into the blood access line in order to increase the pressure in the primary circuit from an initial value to a predetermined pressure;   sensing the pressure in the primary circuit for a predetermined period of time; and either   emitting an alarm if the predetermined pressure is not reached or if the pressure decreases, or   allowing the pressure to decrease if the pressure remains substantially constant for the predetermined period of time.   
     
     
       47. Method according to claim 46, wherein the step of allowing the pressure to decrease in the primary circuit comprises: pumping priming fluid from the blood access line until the pressure in the circuit decreases to the initial value; and   opening the return line.   
     
     
       48. Method according to claim 35 wherein the step of monitoring the pressure in the blood access line comprises: sensing the pressure in the blood access line; and   comparing the pressure sensed to a predetermined first minimum pressure and when the sensed pressure falls below the first minimum pressure, decreasing but not stopping the flow rate of blood by at least one step having a determined duration until: the pressure sensed remains substantially constant for the duration of a step, the flow rate of the blood being then increases until it reaches the initially defined flow rate; or   the pressure sensed decreases for a predetermined period of time or until it reaches a predetermined second minimum pressure, an alarm being then emitted.     
     
     
       49. Method according to claim 35 wherein the step of monitoring the pressure in the blood return line comprises: sensing the pressure in the blood return line; and   comparing the pressure sensed to a predetermined first maximum pressure, and when the sensed pressure raises above the first maximum pressure, decreasing the flow rate of blood by at least one step having a determined duration until: the pressure sensed remains substantially constant for the duration of a step, the flow rate of the blood being then increased until it reaches the initially defined flow rate; or   the pressure sensed increases for a predetermined period of time or until it reaches a predetermined second maximum pressure, an alarm being then emitted.     
     
     
       50. Method according to claim 35, wherein the step of monitoring the pressure in at least one of the blood access line, the blood return line and the collection line comprises: sensing the pressure in the line with a pressure sensor comprising a pressure chamber having a first compartment in fluid communication with the line and a second compartment separated from the first chamber by a flexible membrane;   checking an operative condition of the pressure chamber at regular intervals.   
     
     
       51. Method according to claim 50, wherein the step of checking an operative condition of the pressure chamber comprises: changing instantaneously the flow rate in the line of the chamber;   calculating the change of pressure with respect to time (dp/dt); and   emitting an alarm if dp/dt is less than a predetermined value.   
     
     
       52. Method according to claim 35, wherein the step of monitoring the flow rate of liquid comprises monitoring the flow rate of at least one of the replacement fluid, secondary fluid and anticoagulant fluid by: measuring at regular intervals the actual weight of a container containing the fluid and connected to the corresponding fluid line;   for each interval, calculating a corresponding desired weight from the flow rate defined for the fluid and the treatment time elapsed;   calculating the difference between the actual weight and the desired weight of the container;   regulating the circulation of the fluid in order that the difference between the actual weight and the desired weight of the container tends towards zero.   
     
     
       53. Method according to claim 52, wherein the interval between two weight measurements is chosen as a function of the flow rate defined for the fluid, the faster the flow rate, the shorter the interval. 
     
     
       54. Method according to claim 52, wherein the step of regulating the circulation of the fluid takes into account the previous change in the calculated difference, in order to avoid that the actual flow rate of the fluid oscillate around the defined flow rate. 
     
     
       55. Method according to claim 35, wherein the step of monitoring the flow rate of the collection fluid comprises: measuring at regular intervals the actual weight of a container connected to the collection fluid line,   for each interval, calculating a corresponding desired weight from the flow rate defined for the collection fluid and the treatment time elapsed;   calculating the difference between the actual weight and the desired weight of the container;   regulating the circulation of the collection fluid in order that the difference between the actual weight and the desired weight of the container tends toward zero.